The present invention relates to a hollow blow-molded container of a biaxially oriented polyester resin. More specifically, it relates to a process for manufacturing such a container of hollow bottle shape free from crazing regardless of the type of its contents, in which such sections as neck, neck end and/or bottom sections thereof (where the resin is not substantially subjected to orientation) are protected against thermal deformation.
Polyethylene terephthalate ("PET") has a wide range of applications in the field of containers for foodstuffs, flavoring materials, cosmetics and so on, because it can be molded, by orientation-blowing, into transparent thin-walled containers having high stiffness, impact strength and improved hygienic qualities. At the same time, it has high molding accuracy. In ordinary direct blow molding processes in which the top and bottom parts of an extruded parison are held by a mold and compressed air is blown into the thus held parison to expand the same into a container shape, resultant containers may often have unsatisfactory strength and transparency because the parison is oriented only monoaxially. Therefore, in blow molding, there prevails a so-called biaxially-orienting blow-molding process in which the parison is oriented not only laterally but also longitudinally in a temperature range suitable for such orientation. Biaxially-oriented blow-molded containers show increased stiffness and strength as well as improved gas barrier properties and transparency. However, even in such a biaxially-orienting blow-molding process, such sections as the neck, neck end and bottom of the resultant containers cannot exhibit sufficient improvement in physical properties and are susceptible to thermal deformation, because such sections are not subjected to orientation. That is to say, molecular orientation occurs only to a small (if any) extent, and the bond between the molecular chains constituting polymer crystals is not strong in these sections.
As is well-known, containers which are useful for storing volatile liquids require very tight sealing. Therefore, the neck ends of such containers are usually sealed with a crown cap caulked thereon or a cap placed thereon with interposed packing. However, if the neck section is deformed by some external factor such as heat, etc., the aforementioned sealing effect of the sealing means is diminished. If polyester containers are used to store highly-concentrated alcohol, ester, or cosmetics or solvents containing these, the aforementioned non-oriented sections may be permeated by the contents of the containers to be crazed and, eventually, allow the liquid contents to leak from the neck end of the container. Also, if such containers are filled with carbonated beverages, their bottoms may be bulged out due to high internal pressure and crazing to such an extent as to deteriorate their standing stability, sometimes even causing the containers to burst.
Furthermore, when such containers are used to store such liquids as juice or sauce that are filled thereinto after heat-sterilization, their neck or neck end portions may be deformed by the heat of the heat-sterilized contents to such an extent that the containers cannot be sealed completely even with a cap having an inner sealing seat.